1. Field of the Invention
This invention relates generally to WDM or DWDM systems with optical signal equalization and/or chromatic dispersion (CD) compensation and/or polarization mode dispersion (PMD) compensation for optical signals and more particularly to optical equalizers/dispersion compensators in photonic integrated circuit (PIC) chips.
2. Description of the Related Art
It is known in the art to provide for optical signal equalization as well as dispersion compensation relative to optical signals propagating in a WDM optical communication network. By including optical signal equalization as well as optical signal compensation, albeit chromatic dispersion compensation or polarization mode dispersion or both, in an optical transmission fiber, it provides the possibility for the optical signals to be transmitted a further distance along the transmission fiber without requiring optical signal amplification, such as through a mid-span optical amplifier, for example, with an EDFA. With proper signal equalization and substantial dispersion compensation, it is possible to bypass or skip, i.e., not deploy a mid-span optical-to-electrical-to-optical regenerator (OEO REGEN), thereby costs that would be incurred with an OEO REGEN rather than a less expensive EDFA. This is referred to by some carrier providers and their equipment manufacturers as a “skip-1” feature, wherein with proper addressing of signal equalization and dispersion compensation, the requirement for a mid-span OEO REGEN network node can be “skipped” at least by one or more of such optical amplifiers with proper signal equalization and/or dispersion compensation at a transmission point where such a node would normally be required.
For long-haul transmission, a variation in optical signal amplitude and strength varies over long distances for different wavelengths in accordance with the nonlinearities of the transmission line, i.e., the strength of the different channel signals propagating at different frequencies respectively vary with the transmission loss characteristics of the transmission fiber. Thus, means are provided to compensate for this by providing mid-span signal amplification and equalization. Such equalization systems are taught in U.S. Pat. Nos. 6,487,336; 6,493,502 and 5,933,270 as well as in FIG. 9, for example, of U.S. patent application, Publication No. 2002/0071155, published Jun. 15, 2002.
By the same token, dispersion compensation is provided to the individual optical signals to suppress waveform deterioration of the signals along the transmission fiber, particularly as the rate of transmission is increased. Compensation can be provided by providing an appropriate phase shift to the optical signals, deploying dispersion compensating fibers, deploying bandwidth or linewidth filters or etalons (such as Gires-Toumois etalons), deploying Bragg gratings or providing different path lengths for different optical signals to make corrections either statically or dynamically. Such systems are, in part, disclosed in U.S. Pat. Nos. 4,750,802; 5,473,719; 6,363,184; and 6,137,604 as well as in U.S. patent applications, Publication Nos. 2002/0102052, published Aug. 1, 2002 and 2002/0159701, published Oct. 31, 2002.
All of these examples of equalizers and compensators, except possibly for Publication No. 2002/0102052, are comprised of discrete devices coupled via optical fibers and discrete optical components which render their alignment and final assembly labor intensive forming large size systems that are expensive to manufacture and implement. In Publication No. 2002/0102052, the planar waveguide dispersion compensator illustrates an AWG combined with a path length adjuster employing thermally adjusted lens strips with an array of waveguides in silica that are optically coupled to the AWG. While the planar waveguide dispersion compensator is shown in a single device, a portion of it is external to the device (an index matched medium and a reflector) and the manufacture of such a device would be complex in silicon in forming the AWG/dispersion compensation region as an integrated device. As mentioned in this publication, dispersion is compensated by splitting a respective channel signal into a plurality of component signals having fractional differences in wavelength,
            Δ      ⁢                          ⁢      λ        λ    ,which fractional differences are small compared to the ratio of the wavelength to the maximum group delay length. Then, a small fractional adjustment,
      δϕ    Δϕ    ,is applied to the relative phase of each component signal. An adjustment in the signal phase for each signal by an induced relative phase shift, Δφ, which varies symmetrically outward from the center of the component signal array. This is performed in the path length adjuster where the path lengths may be varied by the application of heat to change the refractive index of the paths traveled by the component signals.
Also, in Publication No. 2002/0102052, an alternative embodiment is illustrated in FIG. 4B wherein a separate reflector is not utilized but instead, the component channel signals are fed into a second AWG having matched characteristics to those of the first AWG. The second AWG can be folded back on itself as an alternative version. In any case, this approach offers new and different optical components to perform the function of dispersion compensation. However, it would be more desirable to utilized more standard compensator components or components that can be easily integrated on a single photonic integrated circuit (PIC) chip.
Reference is also made to FIG. 1 which discloses a WDM channel equalization/compensator device 10 comprising multiple discrete electro-optic (active) and passive optical components. Such a device 10 is illustrated in U.S. Pat. No. 6,487,336, which is incorporated herein by its reference. As shown in FIG. 1, a transmitted multiplexed signal, λ1, λ2, . . . , λN, on an optical span 11 is directed to multiplexer/demultiplexer (MUX/DEMUX) 14 via circulator 12 and optical fiber 13 where the signal is demultiplexed into separate signal channels 15 which number the amount of signal channels, i.e., N channel signals. The separate channel signals are then attenuated by in-fiber variable optical attenuators (VOAs) 16 which are deployed to independently adjust and control the power levels of the channel signals and provide for power equalization across the channel array where equal power in the channels is desired. The signals then pass through Faraday rotators (FRs) 17 where the polarization mode is rotated 45°. Then, the signals are reflected at reflector 18 and again pass through the Faraday rotators (FRs) 17 and VOAs 16 so that the polarization mode of the respective signals are again rotated 45° and the signals are again attenuated for the second time. The signals are then multiplexed at MUX/DEMUX 14 and provided as an output on fiber 13 where they are placed on output fiber span 19. Thus, the Faraday rotation of 90° relative to each channels signal is accomplished so that the reflected signals are essentially orthogonal to the polarization of the original signals such that any polarization-dependent loss (PDL) with respect to two orthogonal polarizations is substantially eliminated. Also, the attenuators 16 are properly controlled to result in the second channel signals to be all approximately the same power.
Of higher desirability is a channel equalizer/compensator that can be included on a single PIC chip that also can perform chromatic dispersion (CD) compensation in addition to, or alternatively to, polarization mode dispersion (PMD).